London Underground has been quick to deny reports that it banned a revealing poster for an art exhibition because there was a naked breast in the picture.

The 17th Century painting of the Countess of Oxford was said to have been judged too risky for tube stations.

The National Portrait Gallery, which is holding the exhibition entitled Painted Ladies, decided it was best that tube travellers see an alternative painting - this time with the subject's assets covered up.

But London Underground maintained it would have been happy to show the countess.

Sir Peter Lely's Duchess of Richmond was chosen for London Underground

Thursday's Guardian carried an article saying it was London Underground who had rejected the poster.

But the train network's press officer Jan O'Neill told BBC News Online: "We were never shown the poster and having now seen it I can say that it is not the sort of thing we would reject.

"For reasons best-known to them, the people who graffiti posters seem happy to draw over a perfectly innocuous picture of a woman in a swimsuit, but when it comes to fine art they ignore it."

She said London Underground does have strict rules over what advertising it will permit, however.

The people who graffiti posters seem happy to draw over a perfectly innocuous picture of a woman in a swimsuit, but when it comes to fine art they ignore it

Jan O'Neill, London Underground

"If you are in the street and you see a poster you don't like you can look away, but if you are on the Underground you are more captive and might be waiting a few minutes in front of a poster," she added.

Hazel Sutherland, from the National Portrait Gallery, said the bare-breasted lady was not submitted to the Underground.

"We were aware of their regulations regarding taste and graffiti," she said.

She added that they created another posted just for the tube, which was also a "strong image" which also featured the subject wearing a yellow dress.

The National Portrait Gallery created the two posters by the artist Sir Peter Lely, for its exhibition, which it is running jointly with the Yale Centre for British Art.

Adverting posters, except those in the underground, will show the painting of Diana Kirk, Countess of Oxford, casually revealing her left breast.

Tube travellers will instead see Frances Stuart, Duchess of Richmond, whose covered up image was deemed more suitable.

Martin Amis' book cover was banned from the underground

London Underground has taken exception to advertising posters in the past.

A poster for author Martin Amis' autobiography was banned because the picture used showed him smoking a cigarette as a child.

The company said the photograph - which appears on the book's cover - could incite children to take up under-age smoking.

Amis's publishers Vintage used a different poster for the train network with a plain background.

The tube authorities also banned two other posters for clothes shop Miss Selfridge.

One showed five women giving the V-sign while the other showed scantily-clad female models, after the Underground decided customers would find it "difficult to avert their eyes".